"Our commercial solution is better supported than open-source alternatives"
Two sides of the same fib. Over time, licensing fees will constitute an increasingly smaller fraction of your total cost of ownership. The really big expenses lie in customization and integration, and here, some open-source tools will cost you more than their commercial equivalents.
At the same time, you'll find that support for a typical commercial CMS compares poorly against the kind of community help you can receive from from a large, global, open-source project. Key word there is "large."
Note that what is important is the size of the user community, not the size of the company.